JP2005025120A - Optical compensation sheet, polarizing plate and liquid crystal display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical compensation sheet using a cellulose acetate film as a component, usable as a protective film of a polarizing plate, and capable of adding an optical compensation function to the polarizing plate without increasing the number of the components of the polarizing plate, to provide a polarizing plate using the optical compensation sheet as a protective film and ensuring an increased yield rate in a polarizing plate working step, and to provide a liquid crystal display in which the polarizing plate is disposed and a liquid crystal cell is optically compensated. <P>SOLUTION: The optical compensation sheet comprises a cellulose acetate film having an acetylation degree of 59.0-61.5%, has been surface-saponified, and has a contact angle of a surface to water of 30-70°. In another aspect, the optical compensation sheet has an optically anisotropic layer comprising a liquid crystal on the above cellulose acetate film. The polarizing plate uses the optical compensation sheet as a transparent protective film. The liquid crystal display device includes the polarizing plate. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical compensation sheet, a polarizing plate using the same, and a liquid crystal display device.

Cellulose acetate film is used for various photographic materials and optical materials because of its toughness and flame retardancy. Cellulose acetate film is a typical support for photographic light-sensitive materials. Cellulose acetate films are also used in liquid crystal display devices. The cellulose acetate film is characterized by high optical isotropy (low retardation value) as compared with other polymer films. Therefore, it is common to use a cellulose acetate film for an application requiring optical isotropy, for example, a polarizing plate.
In contrast, optical anisotropy (high retardation value) is required for an optical compensation sheet (phase difference film) of a liquid crystal display device. Therefore, it is common to use a synthetic polymer film having a high retardation value such as a polycarbonate film or a polysulfone film as the optical compensation sheet.
In addition to the optical compensation sheet made of a synthetic polymer film, an optical compensation sheet is also proposed in which an optically anisotropic layer formed of discotic liquid crystalline molecules is provided on a transparent support (for example, Patent Document 1, (See Patent Document 2, Patent Document 3, and Patent Document 4). The high retardation value required for the optical compensation sheet is achieved by the optically anisotropic layer. On the other hand, since the transparent support is required to have high optical isotropy (low retardation value), a cellulose ester film is usually used.

As described above, in the technical field of optical materials, when a polymer film requires optical anisotropy (high retardation value), a synthetic polymer film is used and optical isotropy (low retardation value) is used. It was a general rule to use cellulose acetate film when required.
Patent Document 5 (European Patent No. 0911656A2) discloses a cellulose acetate film having a high retardation value that can be used for applications requiring optical anisotropy, overcoming the conventional general principle. ing. It is described that a liquid crystal display device with high display quality can be obtained by inserting this cellulose acetate film as a protective film for a polarizing film between the polarizing film and a liquid crystal cell.
However, in the processing step of the polarizing plate, when the protective film is washed with water, the contact angle with water is too low (easy to get wet), water drops remain, and the productivity of the polarizing plate is significantly reduced. was there.

JP-A-3-9325 Japanese Patent Laid-Open No. 6-148429 JP-A-8-50206 JP-A-9-26572 European Patent 0911656A2

An object of the present invention is to use a cellulose acetate film as a constituent element, which can be used as a protective film for a polarizing plate, and can add an optical compensation function to the polarizing plate without increasing the number of constituent elements. An optical compensation sheet is provided.
Another object of the present invention is to provide a polarizing plate in which the optical compensation sheet is used as a protective film and the productivity is not lowered in the processing step of the polarizing plate.
Still another object of the present invention is to provide a liquid crystal display device in which the polarizing plate is provided and the liquid crystal cell is optically compensated.

The object of the present invention is achieved by the following optical compensation sheet, polarizing plate, and liquid crystal display device.
1. An optical film comprising a cellulose acetate film having an acetylation degree of 59.0 to 61.5%, the surface is saponified, and the contact angle of water on the surface is from 30 ° to 70 ° Compensation sheet.
2. A cellulose acetate film support having an acetylation degree of 59.0 to 61.5% has an optically anisotropic layer made of a liquid crystal compound on one side, and one side of the support is saponified. And an optical compensation sheet characterized by having a contact angle of water of 30 ° to 70 ° or less.
3. 3. The optical compensation sheet according to 1 or 2 above, wherein the cellulose acetate film contains 0.01 to 20 parts by mass of an aromatic compound having at least two aromatic rings with respect to 100 parts by mass of cellulose acetate.
4). 4. The optical compensation sheet according to 3 above, wherein the aromatic compound has at least one 1,3,5-triazine ring.
5. The Re retardation value defined by the following formula (I) of the cellulose acetate film is 5 to 70 nm, and the Rth retardation value defined by the following formula (II) is 70 to 400 nm. The optical compensation sheet according to claim 1.
Formula (I): Re = (nx−ny) × d
Formula (II): Rth = {(nx + ny) / 2−nz} × d
[Where nx is the refractive index in the slow axis direction in the film plane; ny is the refractive index in the fast axis direction in the film plane; nz is the refractive index in the thickness direction of the film] And d is the film thickness (nm)].
6). A polarizing plate comprising a polarizing film and two transparent protective films disposed on both sides thereof,
6. A polarizing plate, wherein one of the transparent protective films is the optical compensation sheet according to the above 1 to 5, and the optical compensation sheet is disposed so that the saponification surface of the support is on the polarizing film side. .
7. A liquid crystal display device comprising a liquid crystal cell and two polarizing plates disposed on both sides thereof, the polarizing plate comprising a polarizing film and two transparent protective films disposed on both sides thereof,
The polarizing plate described in 6 above is used as at least one polarizing plate, and the slow axis of the cellulose acetate film of the polarizing plate and the transmission axis of the polarizing film adjacent to the cellulose acetate film are substantially perpendicular, Alternatively, the liquid crystal display device is arranged in parallel.
8). 8. The liquid crystal display device according to 7 above, wherein the liquid crystal cell is an OCB mode, VA mode, or TN mode liquid crystal cell.

The optical compensation sheet using the cellulose acetate film of the present invention can be used as a protective film for a polarizing plate in any of the embodiments, and further optically compensates for the polarizing plate without increasing the number of components in the polarizing plate. Functions can be added. In addition, when this optical compensation sheet is used as a protective film for a polarizing plate, the cellulose acetate film is saponified and has a large contact angle with water, so that it has excellent adhesion to the polarizing film and also includes washing of the polarizing film with water. Water drainage is good in the process, and the yield in the processing process is improved.
The liquid crystal cell provided with the polarizing plate of the present invention is optically compensated, has excellent visibility, and has a wide viewing angle.

The optical compensation sheet, polarizing plate, and liquid crystal display device of the present invention will be described in order.
[Optical compensation sheet]
The optical compensation sheet according to the first aspect of the present invention comprises an optically anisotropic cellulose acetate film having an acetylation degree of 59.0 to 61.5%, the surface is saponified, The optical compensation sheet has a water contact angle of 30 ° to 70 °, preferably 40 ° to 70 °.
The optical compensation sheet of the second aspect of the present invention has an optically anisotropic layer on one surface of a cellulose acetate film support having an acetylation degree of 59.0 to 61.5%. One surface is an optical compensation sheet that has been saponified and has a water contact angle of 30 ° to 70 °, preferably 40 ° to 70 °.
As described above, the present invention is characterized in that the cellulose acetate film used for the optical compensation sheet is saponified, and the contact angle of water is 30 ° or more and 70 ° or less. When the optical compensation sheet of the present invention is applied to a polarizing plate, this saponification treatment improves the adhesion to the polarizing film, and when the contact angle of water is 30 ° or more, the wettability to water is low. After washing with water in the process, no water droplets remain and the yield is improved. On the other hand, when the contact angle of water exceeds 70 °, it becomes easy to repel water, and the attached dirt is difficult to remove.

In the optical compensation sheet of the second aspect, it is preferable to form an alignment film on a cellulose acetate film and provide an optically anisotropic layer containing a discotic compound or a rod-shaped liquid crystal compound thereon. The optically anisotropic layer is formed by aligning a disk-like liquid crystal compound or a rod-like liquid crystal compound on the alignment film and fixing the alignment state. Thus, when providing an optically anisotropic layer on a cellulose acetate film, conventionally, in order to ensure the adhesion between the cellulose acetate film and the alignment film, it was necessary to provide a gelatin undercoat layer between them. The surface treatment described later is performed, for example, by adding an aromatic compound having at least two aromatic rings as described below, so that the contact angle of water is within the above range, thereby making the gelatin subbing layer unnecessary. Can do.
The thickness of the optically anisotropic layer is preferably from 0.1 to 10 μm, more preferably from 0.5 to 5.0 μm.

The above-mentioned disc liquid crystal compound generally has a large birefringence and has various alignment forms, and therefore has optical properties that cannot be obtained with a conventional stretched birefringent film by using a discotic compound. An optical compensation sheet can be manufactured. Optical compensation sheets using a discotic compound are described in JP-A-6-214116, US Pat. Nos. 5,583,679, 5,646,703 and West German Patent 3,911,620A1.
The cellulose acetate film and its surface treatment will be described in detail in the section of [Polarizing plate].

[Polarizer]
The polarizing plate of the present invention comprises a polarizing film and two transparent protective films disposed on both sides thereof, one of which comprises the above optical compensation sheet. Examples of the polarizing film include an iodine polarizing film, a dye polarizing film using a dichroic dye, and a polyene polarizing film. In general, the iodine polarizing film and the dye polarizing film can be manufactured using a polyvinyl alcohol film.
The transparent protective film is surface-treated in order to improve adhesion with the polarizing film. When the polarizing film is polyvinyl alcohol, the protective film is preferably subjected to a hydrophilic treatment. The protective film is preferably dedusted by a water washing step before adhering to the polarizing film. From the viewpoint of remaining water droplets, the contact angle of the protective film surface with water is preferably 30 ° or more. If it is less than 30 °, the entire surface of the protective film remains wet with water, and it becomes very difficult to drain. In particular, it is preferable to control the contact angle of the surface of the protective film on the polarizing film side in view of the optical characteristics of the polarizing plate.
In the present invention, the optical compensation sheet of the present invention using a cellulose acetate film is used for one of the two protective films of the polarizing plate. For the other protective film, a normal cellulose acetate film may be used.
The slow axis of the protective film and the transmission axis of the polarizing film are arranged so as to be substantially parallel.

[Cellulose acetate film]
As the cellulose acetate film used in the present invention, cellulose acetate having an acetylation degree of 59.0 to 61.5% is used.
The degree of acetylation means the amount of bound acetic acid per unit weight of cellulose. The degree of acetylation follows the measurement and calculation of the degree of acetylation in ASTM D-817-91 (test method for cellulose acetate and the like).
The viscosity average degree of polymerization (DP) of cellulose acetate is preferably 250 or more, and more preferably 290 or more.
Moreover, it is preferable that the cellulose acetate used for this invention has the narrow molecular weight distribution shown by Mw / Mn (Mw is a mass average molecular weight and Mn is a number average molecular weight) by gel permeation chromatography. The specific value of Mw / Mn is preferably 1.0 to 5.0, more preferably 1.3 to 3.0, and most preferably 1.4 to 2.0. preferable.

<Manufacture of cellulose acetate film>
It is preferable to produce a cellulose acetate film by a solvent cast method. In the solvent cast method, a film is produced using a solution (dope) in which cellulose acetate is dissolved in an organic solvent.
The organic solvent is a solvent selected from ethers having 3 to 12 carbon atoms, ketones having 3 to 12 carbon atoms, esters having 3 to 12 carbon atoms, and halogenated hydrocarbons having 1 to 6 carbon atoms. It is preferable to contain.
The ether, ketone and ester may have a cyclic structure. A compound having two or more functional groups of ether, ketone and ester (that is, —O—, —CO— and —COO—) can also be used as the organic solvent. The organic solvent may have another functional group such as an alcoholic hydroxyl group. In the case of an organic solvent having two or more types of functional groups, the number of carbon atoms may be within the specified range of the compound having any functional group.

Examples of the ether having 3 to 12 carbon atoms include diisopropyl ether, dimethoxymethane, dimethoxyethane, 1,4-dioxane, 1,3-dioxolane, tetrahydrofuran, anisole and phenetole.
Examples of ketones having 3 to 12 carbon atoms include acetone, methyl ethyl ketone, diethyl ketone, diisobutyl ketone, cyclohexanone and methylcyclohexanone.
Examples of the esters having 3 to 12 carbon atoms include ethyl formate, propyl formate, pentyl formate, methyl acetate, ethyl acetate and pentyl acetate.
Examples of the organic solvent having two or more kinds of functional groups include 2-ethoxyethyl acetate, 2-methoxyethanol and 2-butoxyethanol.
The number of carbon atoms of the halogenated hydrocarbon is preferably 1 or 2, and most preferably 1. The halogen of the halogenated hydrocarbon is preferably chlorine. The proportion of halogenated hydrocarbon hydrogen atoms substituted with halogen is preferably 25 to 75 mol%, more preferably 30 to 70 mol%, and more preferably 35 to 65 mol%. More preferably, it is 40 to 60 mol%, and most preferably. Methylene chloride is a representative halogenated hydrocarbon.
Two or more organic solvents may be mixed and used.

A cellulose acetate solution can be prepared by a general method. The general method means that the treatment is performed at a temperature of 0 ° C. or higher (room temperature or high temperature). The solution can be prepared using a dope preparation method and apparatus in a normal solvent cast method. In the case of a general method, it is preferable to use a halogenated hydrocarbon (particularly methylene chloride) as the organic solvent.
The amount of cellulose acetate is adjusted so that it is contained in an amount of 10 to 40% by mass in the resulting solution. The amount of cellulose acetate is more preferably 10 to 30% by mass. Arbitrary additives described later may be added to the organic solvent (main solvent).
The solution can be prepared by stirring cellulose acetate and an organic solvent at room temperature (0 to 40 ° C.). High concentration solutions may be stirred under pressure and heating conditions. Specifically, cellulose acetate and an organic solvent are placed in a pressure vessel and sealed, and stirred while heating to a temperature not lower than the boiling point of the solvent at normal temperature and in a range where the solvent does not boil. The heating temperature is usually 40 ° C. or higher, preferably 60 to 200 ° C., more preferably 80 to 110 ° C.

Each component may be coarsely mixed in advance and then placed in a container. Moreover, you may put into a container sequentially. The container needs to be configured so that it can be stirred. The container can be pressurized by injecting an inert gas such as nitrogen gas. Moreover, you may utilize the raise of the vapor pressure of the solvent by heating. Or after sealing a container, you may add each component under pressure.
When heating, it is preferable to heat from the outside of the container. For example, a jacket type heating device can be used. The entire container can also be heated by providing a plate heater outside the container and piping to circulate the liquid.
It is preferable to provide a stirring blade inside the container and stir using this. The stirring blade preferably has a length that reaches the vicinity of the wall of the container. A scraping blade is preferably provided at the end of the stirring blade in order to renew the liquid film on the vessel wall.
Instruments such as a pressure gauge and a thermometer may be installed in the container. Each component is dissolved in a solvent in the container. The prepared dope is taken out of the container after cooling, or taken out and then cooled using a heat exchanger or the like.

A solution can also be prepared by a cooling dissolution method. In the cooling dissolution method, cellulose acetate can be dissolved in an organic solvent that is difficult to dissolve by a normal dissolution method. In addition, even if it is a solvent which can melt | dissolve a cellulose acetate with a normal melt | dissolution method, there exists an effect that a uniform solution can be obtained rapidly according to a cooling melt | dissolution method.
In the cooling dissolution method, first, cellulose acetate is gradually added to an organic solvent with stirring at room temperature.
The amount of cellulose acetate is preferably adjusted so as to be contained in the mixture at 10 to 40% by mass. The amount of cellulose acetate is more preferably 10 to 30% by mass. Furthermore, you may add the arbitrary additive mentioned later in a mixture.

The mixture is then cooled to -100 to -10 ° C (preferably -80 to -10 ° C, more preferably -50 to -20 ° C, most preferably -50 to -30 ° C). The cooling can be performed, for example, in a dry ice / methanol bath (−75 ° C.) or a cooled diethylene glycol solution (−30 to −20 ° C.). When cooled in this way, the mixture of cellulose acetate and organic solvent solidifies.
The cooling rate is preferably 4 ° C./min or more, more preferably 8 ° C./min or more, and most preferably 12 ° C./min or more. The faster the cooling rate, the better. However, 10,000 ° C./second is the theoretical upper limit, 1000 ° C./second is the technical upper limit, and 100 ° C./second is the practical upper limit. The cooling rate is a value obtained by dividing the difference between the temperature at the start of cooling and the final cooling temperature by the time from the start of cooling until the final cooling temperature is reached.

Furthermore, when this is heated to 0 to 200 ° C. (preferably 0 to 150 ° C., more preferably 0 to 120 ° C., most preferably 0 to 50 ° C.), cellulose acetate is dissolved in the organic solvent. The temperature can be raised by simply leaving it at room temperature or in a warm bath.
The heating rate is preferably 4 ° C./min or more, more preferably 8 ° C./min or more, and most preferably 12 ° C./min or more. The higher the heating rate, the better. However, 10,000 ° C./second is the theoretical upper limit, 1000 ° C./second is the technical upper limit, and 100 ° C./second is the practical upper limit. The heating rate is a value obtained by dividing the difference between the temperature at the start of heating and the final heating temperature by the time from the start of heating until the final heating temperature is reached. .
A uniform solution is obtained as described above. If the dissolution is insufficient, the cooling and heating operations may be repeated. Whether or not the dissolution is sufficient can be determined by merely observing the appearance of the solution with the naked eye.

In the cooling dissolution method, it is desirable to use a sealed container in order to avoid moisture mixing due to condensation during cooling. In the cooling and heating operation, when the pressure is applied during cooling and the pressure is reduced during heating, the dissolution time can be shortened. In order to perform pressurization and decompression, it is desirable to use a pressure-resistant container.
In addition, according to differential scanning calorimetry (DSC), a 20 mass% solution obtained by dissolving cellulose acetate (acetylation degree: 60.9%, viscosity average polymerization degree: 299) in methyl acetate by a cooling dissolution method was 33. There exists a quasi-phase transition point between a sol state and a gel state in the vicinity of ° C., and a uniform gel state is obtained below this temperature. Therefore, this solution needs to be stored at a temperature equal to or higher than the pseudo phase transition temperature, preferably about the gel phase transition temperature plus about 10 ° C. However, this pseudo phase transition temperature varies depending on the degree of acetylation of cellulose acetate, the degree of viscosity average polymerization, the concentration of the solution, and the organic solvent used.

A cellulose acetate film is produced from the prepared cellulose acetate solution (dope) by a solvent cast method.
The dope is cast on a drum or band, and the solvent is evaporated to form a film. It is preferable to adjust the concentration of the dope before casting so that the solid content is 18 to 35%. The surface of the drum or band is preferably finished in a mirror state. Regarding casting and drying methods in the solvent cast method, U.S. Pat. Nos. 2,336,310, 2,367,603, 2,429,078, 2,429,297, 2,429,978, 2,607,704, 2,739,69, U.S. 2,739,070, British Patent 6,407,331, No. 736892, JP-B Nos. 45-4554, 49-5614, JP-A-60-176834, No. 60-203430, and No. 62-1115035.
The dope is preferably cast on a drum or band having a surface temperature of 10 ° C. or less. After casting, it is preferable to dry it by applying air for 2 seconds or more. The obtained film can be peeled off from the drum or band, and further dried by high-temperature air whose temperature is successively changed from 100 to 160 ° C. to evaporate the residual solvent. The above method is described in Japanese Patent Publication No. 5-17844. According to this method, it is possible to shorten the time from casting to stripping. In order to carry out this method, it is necessary for the dope to gel at the surface temperature of the drum or band during casting.

A plasticizer can be added to the cellulose acetate film in order to improve mechanical properties or increase the drying rate. As the plasticizer, phosphoric acid ester or carboxylic acid ester is used. Examples of phosphate esters include triphenyl phosphate (TPP) and tricresyl phosphate (TCP). Representative examples of the carboxylic acid ester include phthalic acid esters and citric acid esters. Examples of phthalic acid esters include dimethyl phthalate (DMP), diethyl phthalate (DEP), dibutyl phthalate (DBP), dioctyl phthalate (DOP), diphenyl phthalate (DPP) and diethylhexyl phthalate (DEHP). Examples of citrate esters include triethyl O-acetylcitrate (OACTE) and tributyl O-acetylcitrate (OACTB). Examples of other carboxylic acid esters include butyl oleate, methylacetyl ricinoleate, dibutyl sebacate, and various trimellitic acid esters. Phthalate plasticizers (DMP, DEP, DBP, DOP, DPP, DEHP) are preferably used. DEP and DPP are particularly preferred.
The addition amount of the plasticizer is preferably 0.1 to 25% by mass of the amount of cellulose acetate, more preferably 1 to 20% by mass, and most preferably 3 to 15% by mass.

  A degradation inhibitor (eg, antioxidant, peroxide decomposer, radical inhibitor, metal deactivator, acid scavenger, amine) may be added to the cellulose acetate film. The deterioration preventing agents are described in JP-A-3-199201, JP-A-51907073, JP-A-5-194789, JP-A-5-271471, and JP-A-6-107854. The addition amount of the deterioration preventing agent is preferably 0.01 to 1% by mass of the solution (dope) to be prepared, and more preferably 0.01 to 0.2% by mass. When the addition amount is less than 0.01% by mass, the effect of the deterioration preventing agent is hardly recognized. When the added amount exceeds 1% by mass, bleeding-out (bleeding) of the deterioration preventing agent to the film surface may be observed. Examples of particularly preferred deterioration inhibitors include butylated hydroxytoluene (BHT) and tribenzylamine (TBA).

The cellulose acetate film can be made into an optically anisotropic film by further adjusting the retardation by a stretching treatment. In the first aspect of the present invention, it is essential that the cellulose acetate film has optical anisotropy. The draw ratio is preferably 3 to 100%.
The thickness of the cellulose acetate film is preferably 20 to 120 μm, and more preferably 30 to 100 μm. Most preferred is 40 to 80 μm.

<Saponification treatment of cellulose acetate film>
In the present invention, saponification is performed as the surface treatment, and plasma treatment, flame treatment, and ultraviolet irradiation treatment are performed as necessary.
Saponification treatment includes acid saponification treatment and alkali saponification treatment. Plasma treatment includes corona discharge treatment and glow discharge treatment. In order to maintain the flatness of the film, in these surface treatments, the temperature of the cellulose acetate film is preferably set to a glass transition temperature (Tg) or lower, specifically 150 ° C. or lower.

The alkali saponification treatment is preferably performed in a cycle in which the cellulose acetate film is immersed in an alkali solution, neutralized with an acidic solution, washed with water and dried.
Examples of the alkaline solution include potassium hydroxide solution and sodium hydroxide solution. The specified concentration of hydroxide ions in the alkaline solution is preferably 0.1N to 3.0N, more preferably 0.5N to 2.0N. The temperature of the alkaline solution is preferably in the range of 0 to 90 ° C, more preferably 40 to 70 ° C.

In the corona discharge treatment, a high voltage is applied between the electrode connected to the high voltage generator and the dielectric roll, and the cellulose acetate film is placed or moved during the corona discharge generated between the electrode and the dielectric roll. Is done. In this specification, the frequency of the high voltage applied between the electrode and the dielectric roll is referred to as the discharge frequency.
Corona discharge treatment is easy to perform in the atmosphere, but if necessary, the treatment device is sealed or semi-sealed and filled with other gas or mixed with other gas and the atmosphere. You can also Examples of the gas include nitrogen gas, argon gas, oxygen gas and the like.
In the corona discharge treatment, the discharge frequency is generally 50 Hz to 5000 kHz, and more preferably 5 kHz to several hundred kHz. In the corona treatment, if the discharge frequency is too low, the discharge becomes unstable and pinholes are generated in the cellulose acetate film, which is not preferable. Also, if the discharge frequency is too high, an additional device for impedance matching is required, which increases the price of the device, which is not preferable.
For normal wettability improvement, the corona discharge treatment of the cellulose acetate film is preferably 0.001 kV · A · min / m ^{2 to 5} kV · A · min / m ^2, and 0.01 kV · A · and even more preferably to a minute / m ² ~1kV · a · min / m ^2. The distance between the electrode and the dielectric roll is preferably 0.5 to 2.5 mm, and more preferably 1.0 to 2.0 mm.

The glow discharge treatment is performed by applying or applying a high voltage between a pair of electrodes in a low-pressure gas and placing or moving the cellulose acetate film in the glow discharge generated between the electrodes.
In the glow discharge treatment, the gas pressure is generally in the range of 0.005 to 20 Torr, and more preferably in the range of 0.02 to 2 Torr. If the pressure is too low, the surface treatment effect is reduced. If the pressure is too high, an excessive current flows, sparking is likely to occur, and the cellulose acetate film may be destroyed. The discharge is generated by applying a high voltage between metal plates or metal bars arranged in a vacuum tank with a pair of spaces or more. This voltage can take various values depending on the composition and pressure of the atmospheric gas, but usually a stable steady glow discharge occurs between 500 and 5000 V within the above pressure range.
In order to improve adhesiveness, it is preferable to set the range of applied voltage to 2000 to 4000V. Moreover, a general discharge frequency is several thousand MHz from direct current, and preferably 50 Hz to 20 MHz. In order to obtain a desired adhesive strength, the glow discharge treatment of the workpiece is preferably 0.01 kV · A · min / m ^{2 to 5} kV · A · min / m ² , preferably 0.15 kV · A · min / m ^2. More preferably, m ^{2 to 1} kV · A · min / m ² is set.

The ultraviolet irradiation treatment is performed by irradiating the cellulose acetate film with ultraviolet rays. In the ultraviolet irradiation treatment, if the surface temperature of the film rises to around 150 ° C. and there is no problem in performance, a high-pressure mercury lamp having a dominant wavelength of 365 nm can be used as the light source. When low temperature treatment is required, it is preferable to use a low pressure mercury lamp having a dominant wavelength of 254 nm. It is also possible to use an ozone-less high-pressure mercury lamp and a low-pressure mercury lamp. Regarding the amount of processed light, the greater the amount of processed light, the better the adhesive force between the film and the adherend layer. However, as the amount of light increases, the film becomes colored and the film becomes brittle. Therefore, when using a high pressure mercury lamp for a 365nm main wavelength, the irradiation light amount is preferably 20~10000 (mJ / cm ^2), more preferably 50~2000 (mJ / cm ^2). When using a low pressure mercury lamp for a 254nm main wavelength, the irradiation light amount is preferably 100~10000 (mJ / cm ^2), more preferably 300~1500 (mJ / cm ^2).

  Specifically, the contact angle of water on the surface of the optical compensation sheet (cellulose acetate film) of the present invention is determined by dropping surface pure water onto the cellulose acetate film, The angle between the drawn tangent line and the film surface is measured by defining the angle containing the droplet as the contact angle.

Even when the cellulose acetate film is subjected to the above saponification treatment, there may be a problem in adhesion to the polarizing film (problem such as peeling from the polarizing film in the durability test).
In order to improve the contact angle of water on the surface of the protective film between 30 ° and 70 ° and to improve the adhesion to the polarizing film, the following hydrophobic compound (aromatic compound having two aromatic rings) Is preferably added to the cellulose acetate film.

<Aromatic compound having two aromatic rings>
In order to adjust the contact angle and retardation of the cellulose acetate film, an aromatic compound having at least two aromatic rings is used as an additive. Hereinafter, this compound is also called a retardation adjusting agent.
The aromatic compound is used in the range of 0.01 to 20 parts by mass with respect to 100 parts by mass of cellulose acetate. The aromatic compound is preferably used in the range of 0.05 to 15 parts by mass, more preferably in the range of 0.1 to 10 parts by mass with respect to 100 parts by mass of cellulose acetate. Two or more aromatic compounds may be used in combination.
The aromatic ring of the aromatic compound includes an aromatic hetero ring in addition to the aromatic hydrocarbon ring.
The aromatic compound is preferably highly hydrophobic from the viewpoint of contact angle control.
The aromatic ring (aromatic hydrocarbon ring, aromatic heterocycle) of the aromatic compound used in the present invention is disclosed in paragraphs [0011] to [0085] of JP-A No. 2000-1111914.
The molecular weight of the retardation increasing agent is preferably 300 to 800.

<Retardation of cellulose acetate film>
The Re retardation value and Rth retardation value of the film are defined by the following formulas (I) and (II), respectively.
Formula (I): Re = (nx−ny) × d
Formula (II): Rth = {(nx + ny) / 2−nz} × d
In formulas (I) and (II);
nx is the refractive index in the slow axis direction (the direction in which the refractive index is maximum) in the film plane.
ny is the refractive index in the fast axis direction (the direction in which the refractive index is minimum) in the film plane.
nz is the refractive index in the thickness direction of the film.
d is the thickness of the film whose unit is nm.

In the present invention, it is preferable to adjust the Re retardation value of the cellulose acetate film to 20 to 70 nm and the Rth retardation value to 70 to 400 nm.
When two optically anisotropic cellulose acetate films (optical compensation sheets) are used in the liquid crystal display device, the Rth retardation value of the film is preferably 70 to 250 nm.
When one optically anisotropic cellulose acetate film (optical compensation sheet) is used for the liquid crystal display device, the Rth retardation value of the film is preferably 150 to 400 nm.
The birefringence (Δn: nx−ny) of the cellulose acetate film is preferably 0.00025 to 0.00088. The birefringence {(nx + ny) / 2−nz} in the thickness direction of the cellulose acetate film is preferably 0.00088 to 0.005.

[Liquid Crystal Display]
The optical compensation sheet of the present invention or the polarizing plate using the optical compensation sheet is advantageously used for a liquid crystal display device, particularly a transmissive liquid crystal display device.
The transmissive liquid crystal display device includes a liquid crystal cell and two polarizing plates disposed on both sides thereof. The liquid crystal cell carries a liquid crystal between two electrode substrates.
One optical compensation sheet is disposed between the liquid crystal cell and one polarizing plate, or two optical compensation sheets are disposed between the liquid crystal cell and both polarizing plates.

Of the two polarizing plates, at least one of the polarizing plates uses an optical compensation sheet using the above cellulose acetate film as a transparent protective film disposed between the liquid crystal cell and the polarizing film. Any polarizing plate may use an optical compensation sheet as the transparent protective film.
The liquid crystal cell is preferably in OCB mode, VA mode or TN mode.

  The OCB mode liquid crystal cell is a liquid crystal display device using a bend alignment mode liquid crystal cell in which rod-like liquid crystalline molecules are aligned in a substantially opposite direction (symmetrically) between the upper part and the lower part of the liquid crystal cell. It is disclosed in the specifications of Japanese Patent Nos. 45882525 and 5410422. Since the rod-like liquid crystal molecules are aligned symmetrically between the upper part and the lower part of the liquid crystal cell, the bend alignment mode liquid crystal cell has a self-optical compensation function. Therefore, this liquid crystal mode is also called an OCB (Optically Compensatory Bend) liquid crystal mode. The bend alignment mode liquid crystal display device has an advantage of high response speed.

  In a VA mode liquid crystal cell, rod-like liquid crystalline molecules are aligned substantially vertically when no voltage is applied. The VA mode liquid crystal cell includes (1) a narrowly defined VA mode liquid crystal cell in which rod-like liquid crystalline molecules are aligned substantially vertically when no voltage is applied, and substantially horizontally when a voltage is applied (Japanese Patent Laid-Open No. Hei 2-). 176625) (2) Liquid crystal cell (SID97, Digest of tech. Papers (Preliminary Proceed) 28 (1997) 845 in which the VA mode is converted into a multi-domain (MVA mode) for widening the viewing angle ), (3) Liquid crystal cell in a mode (n-ASM mode) in which rod-like liquid crystalline molecules are substantially vertically aligned when no voltage is applied and twisted multi-domain alignment is applied when a voltage is applied (1998)) and (4) SURVAVAL mode liquid crystal cells (announced at LCD International 98).

  In a TN mode liquid crystal cell, rod-like liquid crystalline molecules are substantially horizontally aligned when no voltage is applied, and are twisted and aligned at 60 to 120 °. The TN mode liquid crystal cell is most frequently used as a color TFT liquid crystal display device, and is described in many documents.

  As described above, the present inventors improved the cellulose acetate film disclosed in the specification of European Patent 0911656A2, and maintained the conventional thickness or reduced the thickness even if the thickness was further reduced. The liquid crystal cell was optically compensated for without any problems. By adjusting the type and amount of additives (specifically, aromatic compounds having two aromatic rings) to the cellulose acetate film or the production conditions (for example, film stretching conditions), the Re retardation value can be reduced. A cellulose acetate film having a thickness of 20 to 70 nm and an Rth retardation value of 70 to 400 nm is obtained. This cellulose acetate film has sufficient optical anisotropy to optically compensate the liquid crystal cell. Further, by subjecting the cellulose acetate film to a surface treatment, an optical compensation sheet comprising a cellulose acetate film having a contact angle with respect to water of 30 ° to 70 ° and optimal for polarizing plate processing can be obtained.

The polarizing plate includes a polarizing film and protective films disposed on both sides thereof. The polarizing film is obtained by adsorbing iodine or a dichroic dye on a stretched and oriented polyvinyl alcohol, and the protective film is generally made of cellulose acetate film. When the cellulose acetate film is used as one protective film of the polarizing plate, an optical compensation function can be added to the polarizing plate without increasing the number of components of the polarizing plate. At this time, in the present invention, since the contact angle of water on the surface of the cellulose acetate film is 30 ° or more, the yield in the polarizing plate processing step and the adhesiveness to the polarizing film can be compatible.
When cellulose acetate having an acetylation degree of less than 59.0 is used, the above optical anisotropy can be easily achieved, but the physical properties as a cellulose acetate film are lowered. In the present invention, cellulose acetate having an acetylation degree of 59.0 to 61.5% is used, and by achieving the above retardation value by other means (adjustment of the above additives and production conditions), A cellulose acetate film excellent in both optical anisotropy and physical properties and excellent in adhesion to a polarizing film is obtained.
An optical compensation sheet comprising the above cellulose acetate film and a polarizing plate using the above cellulose acetate film as a protective film are an OCB (Optically Compensatory Bend) type, VA (Vertically Aligned) type or TN (Twisted Nematic) type liquid crystal display. It can be used with particular advantage in the apparatus.

[Example 1]
The following composition was put into a mixing tank and stirred while heating to dissolve each component to prepare a cellulose acetate solution.

Cellulose acetate solution composition Cellulose acetate with an acetylation degree of 60.8% 100 parts by weight Triphenyl phosphate (plasticizer) 11.7 parts by weight Biphenyl diphenyl phosphate (plasticizer) 5.85 parts by weight Methylene chloride (first solvent) 300 parts by weight Methanol (second solvent) 54 parts 1-butanol (third solvent) 11 parts

In another mixing tank, 16 parts by mass of the following retardation increasing agent, 80 parts by mass of methylene chloride and 20 parts by mass of methanol were added and stirred while heating to prepare a retardation increasing agent solution.
A dope was prepared by mixing 474 parts by mass of the cellulose acetate solution with 25 parts by mass of the retardation increasing agent solution and stirring sufficiently. The addition amount of the retardation increasing agent was 3.5 parts by mass with respect to 100 parts by mass of cellulose acetate.

The obtained dope was cast using a band casting machine. A cellulose acetate film (thickness: 80 μm) was produced by laterally stretching a film having a residual solvent amount of 15% by mass at a stretching ratio of 25% using a tenter under the conditions of 130 ° C.
Further, the produced cellulose acetate film was immersed in a 1.5N potassium hydroxide solution (50 ° C.) for 2 minutes, and then neutralized with sulfuric acid. The cellulose acetate film was taken out of the solution, washed with pure water, and dried. The contact angle of water on the surface of the cellulose acetate film thus saponified was determined to be 40 °.
About the produced cellulose acetate film (optical compensation sheet), the Re retardation value and the Rth retardation value at a wavelength of 550 nm were measured using an ellipsometer (M-150, manufactured by JASCO Corporation). The results are shown in Table 1.

[Example 2]
A dope was prepared by mixing 474 parts by mass of a cellulose acetate solution with 56 parts by mass of a retardation increasing agent solution (use 7.8 parts by mass of a retardation increasing agent with respect to 100 parts by mass of cellulose acetate), and adjusting the draw ratio. A cellulose acetate film (optical compensation sheet) was produced in the same manner as in Example 1 except that the content was changed to 14%.
Further, the produced cellulose acetate film was immersed in a 1.5N potassium hydroxide solution (40 ° C.) for 5 minutes, and then neutralized with sulfuric acid. The cellulose acetate film was taken out of the solution, washed with pure water, and dried. The contact angle of water on the surface of the cellulose acetate film thus saponified was determined to be 55 °.
For the produced cellulose acetate film (optical compensation sheet), the Re retardation value and the Rth retardation value were measured in the same manner as in Example 1. The results are shown in Table 1.

[Example 3]
A dope is prepared by mixing 474 parts by mass of the cellulose acetate solution with 35 parts by mass of the retardation increasing agent solution (4.8 parts by mass of the retardation increasing agent is used with respect to 100 parts by mass of cellulose acetate), and the draw ratio is set. A cellulose acetate film was produced in the same manner as in Example 2 except that the content was changed to 28%.
Further, a 1.0 N potassium hydroxide solution (solvent: water / isopropyl alcohol / propylene glycol) was applied to the produced cellulose acetate film so as to be 18 cc / m ² and heated to 40 ° C. so as not to be completely dried. After warming, the alkaline solution was removed with pure water after about 20 seconds. The contact angle of water on the surface of the cellulose acetate film thus treated with alkali was found to be 58 °.
On the surface of the cellulose acetate film subjected to alkali treatment, a coating solution having the following composition was applied at 28 ml / m ² with a # 16 wire bar coater. Drying was performed with warm air of 60 ° C. for 60 seconds and further with warm air of 90 ° C. for 150 seconds.
Next, the film formed was rubbed in the direction of 45 ° with the slow axis of the cellulose acetate film (measured at a wavelength of 632.8 nm).

Alignment film coating solution composition Modified polyvinyl alcohol 10.0 parts by weight Water 371.0 parts by weight Methanol 119.0 parts by weight Glutaraldehyde 0.5 parts by weight

(Formation of optically anisotropic layer)
On the alignment film, 41.01 g of the following discotic (liquid crystalline) compound, 4.06 g of ethylene oxide-modified trimethylolpropane triacrylate (V # 360, manufactured by Osaka Organic Chemical Co., Ltd.), cellulose acetate butyrate (CAB551- 0.2, manufactured by Eastman Chemical Co., Ltd.) 0.90 g, cellulose acetate butyrate (CAB531-1, manufactured by Eastman Chemical Co., Ltd.) 0.23 g, photopolymerization initiator (Irgacure 907, manufactured by Ciba Geigy Co.) 1.35 g, 102 g of sensitizer (Kayacure DETX, Nippon Kayaku Co., Ltd.) 0.45 g, citric acid ester (a mixture of citric acid, citric acid monoethyl ester, citric acid diethyl ester, citric acid triethyl ester) Apply the coating solution dissolved in methyl ethyl ketone to the # 3.6 wire. It was applied in the over. This was affixed to a metal frame and heated in a thermostatic chamber at 130 ° C. for 2 minutes to orient the discotic compound. Next, using a 120 W / cm high-pressure mercury lamp at 100 ° C., UV irradiation was performed for 1 minute to polymerize the discotic compound. Then, it stood to cool to room temperature. In this way, an optically anisotropic layer was formed.
For the produced cellulose acetate film (optical compensation sheet), the Re retardation value and the Rth retardation value were measured in the same manner as in Example 1. The results are shown in Table 1.

[Comparative Example 1]
A cellulose acetate film (optical compensation sheet) was prepared in the same manner as in Example 1 except that the cellulose acetate solution was directly used as a dope and the stretching treatment and the saponification treatment were not performed. The foundation value was measured. The results are shown in Table 1.

[Example 4]
A polarizing film was prepared by adsorbing iodine to the stretched polyvinyl alcohol film, and the cellulose acetate film prepared in Example 1 was attached to one side of the polarizing film using a polyvinyl adhesive. A commercially available cellulose acetate film (Fujitac TD80UF, manufactured by Fuji Photo Film Co., Ltd.) was subjected to saponification treatment and attached to the opposite side of the polarizing film using a polyvinyl alcohol-based adhesive.
The contact angle with respect to water of the surface after saponification of a commercially available cellulose acetate film was 18 °.
The transmission axis of the polarizing film and the slow axis of the cellulose acetate film prepared in Example 1 were arranged in parallel. The transmission axis of the polarizing film and the slow axis of the commercially available cellulose triacetate film were arranged so as to be orthogonal to each other.
In this way, a polarizing plate was produced.

[Example 5]
A polarizing plate was produced in the same manner as in Example 4 except that the cellulose acetate film produced in Example 2 was used.

[Example 6]
The optical compensation sheet prepared in Example 3 was immersed in an aqueous 1.5N sodium hydroxide solution (50 ° C.) for 2 minutes, and then neutralized with sulfuric acid. The cellulose acetate film was taken out of the solution, washed with pure water, and dried. The contact angle of water on the surface of the cellulose acetate film thus saponified (the surface on the side without the discotic compound layer) was determined to be 40 °.
A polarizing plate was produced in the same manner as in Example 4 except for the above.

[Example 7]
A pair of polarizing plates and a pair of optical compensation sheets provided in a liquid crystal display device (VL-1530S, manufactured by Fujitsu Limited) using a vertical alignment type liquid crystal cell were peeled off, and the polarizing plate prepared in Example 4 was used instead. Were attached to the observer side and the backlight side one by one through an adhesive so that the cellulose acetate film produced in Example 1 was on the liquid crystal cell side. The crossed Nicols were arranged so that the transmission axis of the polarizing plate on the viewer side was in the vertical direction and the transmission axis of the polarizing plate on the backlight side was in the horizontal direction.
About the produced liquid crystal display device, the viewing angle was measured in eight steps from black display (L1) to white display (L8) using the measuring machine (EZ-Contrast160D, ELDIM company make). The results are shown in Table 2.

[Example 8]
A pair of polarizing plates and a pair of optical compensation sheets provided in a liquid crystal display device (VL-1530S, manufactured by Fujitsu Limited) using a vertical alignment type liquid crystal cell were peeled off, and the polarizing plate prepared in Example 5 was used instead. Was attached to the observer side through an adhesive so that the cellulose acetate film produced in Example 2 was on the liquid crystal cell side. In addition, a commercially available polarizing plate (HLC2-5618HCS, manufactured by Sanritz Corporation) was attached to the backlight side. The crossed Nicols were arranged so that the transmission axis of the polarizing plate on the viewer side was in the vertical direction and the transmission axis of the polarizing plate on the backlight side was in the horizontal direction.
About the produced liquid crystal display device, the viewing angle was measured in eight steps from black display (L1) to white display (L8) using the measuring machine (EZ-Contrast160D, ELDIM company make). The results are shown in Table 2.

[Comparative Example 2]
About a liquid crystal display device (VL-1530S, manufactured by Fujitsu Limited) using a vertical alignment type liquid crystal cell, from a black display (L1) to a white display (L8) using a measuring device (EZ-Contrast160D, manufactured by ELDIM). The viewing angle was measured in 8 stages. The results are shown in Table 2.

[Example 9]
(Preparation of bend alignment liquid crystal cell)
A polyimide film was provided as an alignment film on a glass substrate with an ITO electrode, and the alignment film was rubbed. The obtained two glass substrates were faced to each other so that the rubbing directions were parallel to each other, and the cell gap was set to 6 μm. A liquid crystal compound having a Δn of 0.1396 (ZL1132, manufactured by Merck & Co., Inc.) was injected into the cell gap to produce a bend alignment liquid crystal cell.
Two polarizing plates produced in Example 6 were attached so as to sandwich the produced bend alignment cell. The optically anisotropic layer of the polarizing plate faced the cell substrate, and the rubbing direction of the liquid crystal cell and the rubbing direction of the optically anisotropic layer facing it were antiparallel. A rectangular wave voltage of 55 Hz was applied to the liquid crystal cell. A normally white mode of 2V white display and 5V black display was set. Using the measuring device (EZ-Contrast160D, manufactured by ELDIM) as the contrast ratio of the transmittance ratio (white display / black display), the viewing angle can be adjusted in eight steps from black display (L1) to white display (L8). It was measured. Table 3 shows the measurement results.

[Example 10]
A pair of polarizing plates provided in a liquid crystal display device (6E-A3, manufactured by Sharp Corporation) using a TN type liquid crystal cell is peeled off, and a polarizing plate produced in Example 4 is produced in Example 1 instead. The obtained cellulose acetate film was attached to the viewer side and the backlight side one by one through an adhesive so that the cellulose acetate film was on the liquid crystal cell side. The transmission axis of the polarizing plate on the observer side and the transmission axis of the polarizing plate on the backlight side were arranged so as to be orthogonal to each other.
About the produced liquid crystal display device, the viewing angle was measured in eight steps from black display (L1) to white display (L8) using the measuring machine (EZ-Contrast160D, ELDIM company make). The results are shown in Table 4.

[Comparative Example 3]
About a liquid crystal display device (6E-A3, manufactured by Sharp Corporation) using a TN type liquid crystal cell, from a black display (L1) to a white display (L8) using a measuring device (EZ-Contrast 160D, manufactured by ELDIM) The viewing angle was measured in 8 stages. The results are shown in Table 4.

Claims (8)

  An optical film comprising a cellulose acetate film having an acetylation degree of 59.0 to 61.5%, the surface is saponified, and the contact angle of water on the surface is from 30 ° to 70 ° Compensation sheet.   A cellulose acetate film support having an acetylation degree of 59.0 to 61.5% has an optically anisotropic layer made of a liquid crystal compound on one side, and one side of the support is saponified. And an optical compensation sheet, wherein the contact angle of water is 30 ° or more and 70 ° or less.   The optical compensation sheet according to claim 1 or 2, wherein the cellulose acetate film contains 0.01 to 20 parts by mass of an aromatic compound having at least two aromatic rings with respect to 100 parts by mass of cellulose acetate.   The optical compensation sheet according to claim 3, wherein the aromatic compound has at least one 1,3,5-triazine ring. The Re retardation value defined by the following formula (I) of the cellulose acetate film is 5 to 70 nm, and the Rth retardation value defined by the following formula (II) is 70 to 400 nm. The optical compensation sheet according to claim 1.
Formula (I): Re = (nx−ny) × d
Formula (II): Rth = {(nx + ny) / 2−nz} × d
[Where nx is the refractive index in the slow axis direction in the film plane; ny is the refractive index in the fast axis direction in the film plane; nz is the refractive index in the thickness direction of the film] And d is the film thickness (nm)]. A polarizing plate comprising a polarizing film and two transparent protective films disposed on both sides thereof,
A polarizing film characterized in that one of the transparent protective films is the optical compensation sheet according to claim 1, and the optical compensation sheet is disposed so that the saponification surface of the support is on the polarizing film side. Board. A liquid crystal display device comprising a liquid crystal cell and two polarizing plates disposed on both sides thereof, the polarizing plate comprising a polarizing film and two transparent protective films disposed on both sides thereof,
The polarizing plate according to claim 6 is used for at least one polarizing plate, and the slow axis of the cellulose acetate film of the polarizing plate and the transmission axis of the polarizing film adjacent to the cellulose acetate film are substantially perpendicular, Alternatively, the liquid crystal display device is arranged in parallel.   The liquid crystal display device according to claim 7, wherein the liquid crystal cell is an OCB mode, VA mode, or TN mode liquid crystal cell.